This invention relates to a method for imparting color to a contact lens. More specifically, it relates to an improved method for uniformly dispersing a dye throughout a soft, hydrogel contact lens.
The conventional method for imparting an evenly dispersed tint in a soft contact lens is described, for example, in U.S. Pat. No. 4,468,229. Generally, the lens is first soaked in an aqueous solution of the dye, and then the dye is bonded to the lens in a separate solution. The lens is typically composed of a hydrophilic polymer derived from the polymerization of hydrophilic monomers. The bonding of the dye to the lens is carried out by contacting the soaked lens with an aqueous base prior to the final hydration step, which is intended to provide the soft, hydrogel lens with the desired amount of water at an acceptable pH.
The dyes which are used in the conventional method are typically derived from a halotriazine such as a dihalotriazine or monohalotriazine, especially water-soluble dichlorotriazines. Dichlorotriazine or monohalotriazine dyes that carry sulfonate functionalities, for example, are soluble in water, so it is necessary that bonding occur with the hydrophilic polymer from which the lens is composed before the final hydration step. Otherwise, the dye could migrate within the lens to create an uneven dispersion, or leach out from the lens into the eye of the wearer.
The dye which imparts the tint to a soft lens made using the conventional method not only is dispersed in the lens, but also does not migrate within the lens or leach out of the lens after the bond has formed. The tinted lens is also stable in an aqueous medium and after repeated high temperature cycling, conditions which are present during routine wear and cleaning. The conventional method requires that the lens be soaked in a solution containing the dye which is at a specific concentration of the dye and at a specific conductivity so that the dye diffuses into the polymer. The conductivity is important since one may control the swelling of a lens by selecting various salt concentrations. It is also important that the dye concentration and time the lens stays in the dye soak be precisely controlled since the diffusion kinetics controls the intensity of the tinted contact lens. The conventional method always needs to have a high concentration of dye in the dye wash so that the continuous tinting can be managed. Unfortunately, the conventional method is cumbersome and requires multiple steps, especially at commercial scale production, because it is necessary to soak the lens in a solution of the dye at a specific concentration and time to create a dispersion of the dye in the lens. Therefore, because of this difficulty, alternative methods have been sought.
U.S. Pat. No. 4,157,892 discloses adding a functionality to the polymer from which the lens is derived which is reactive with the dye. The functionalized polymer is prepared by reacting a "coupler monomer" with a conventional hydrophilic monomer. This coupler monomer has a high probability of changing the physical properties of the polymer. The lens prepared from the functionalized polymer is immersed in a solution of a diazonium dye, where the dye then bonds to the polymer. Although adequate bonding occurs, this method still requires immersion of the finished lens in a solution of the dye.
Another interesting method for imparting color to a soft lens is disclosed in U.S. Pat. No. 4,640,805. This patent describes preparing a tinted lens using a conventional spin casting technique. A suspension of dye pigment in liquid monomer is applied to the mold surface prior to polymerization of bulk monomer in the spin cast mold. Although this method provides a simple way for imparting color to the surf ace of the lens, it does require that the mold be stamped or printed with specific geometries and spacing.
Attempts have been made to incorporate the dye in the lens by polymerizing the hydrophilic monomer from which the lens is derived in the presence of the dye. For example, U.S. Pat. No. 4,252,421 discloses polymerizing a hydrophilic monomer in the presence of a water-insoluble dye. The dye is supposed to become entrapped in the finished, hydrated lens because of its incompatibility with water. Unfortunately, the dye will leach out of a lens derived from polymerizing the most commonly used hydrophilic monomer, hydroxyethylmethacrylate (HEMA), when the lens is fully hydrated to greater than about 40 weight percent water.
Both U.S. Pat. No. 4,252,421 and European Patent Application 0 396 376 disclose the use of a dye which is functionalized with a polymerizable group to facilitate bonding of the UV-active monomeric dye during polymerization of the hydrophilic monomer. Although this technique may ensure adequate dye bonding if the reaction between monomer and dye is essentially complete, it requires the step of functionalizing the dye prior to polymerization. Additionally, the dye might alter physical properties if it slows kinetics.
Finally, another attempt to impart color to a contact lens is disclosed in U.S. Pat. No. 4,639,105. This patent discloses spin casting a mixture of liquid monomer, soluble dye and pigment particles to prepare a lens with variations in color achieved by migration of the pigment particles during spin casting. Although this patent indicates that the dyes do not migrate, no mention of what specific dyes are used, and it is believed that such dyes will indeed migrate or leach during wear unless the dye used is functionalized with polymerizable groups as described above. Furthermore, such a lens is unsuitable for those applications where a uniform dispersion of dye or colorant is necessary or desired.
In view of the deficiencies of the prior art, a method of uniformly dispersing a water-soluble dye throughout a soft contact lens, without requiring the step of immersing the finished lens in a solution of the dye or functionalizing the dye before use, is needed. Additionally, such a method would be extremely desirable if it could be used to prepare a tinted contact lens with physical and optical properties which substantially equal those of a conventional untinted lens.